adrenal medulla

adrenal medulla
Name	Adrenal medulla
Latin	medulla glandulae suprarenalis
Caption	The adrenal gland showing the medulla.
System	Endocrine system
Artery	Superior suprarenal artery, Middle suprarenal artery, Inferior suprarenal artery
Vein	Suprarenal veins
Nerve	Preganglionic sympathetic fibers (Splanchnic nerves)

adrenal medulla. The adrenal medulla is the inner part of the adrenal gland, situated atop each kidney. It functions as a specialized sympathetic ganglion and is a crucial component of the body's neuroendocrine system. Composed primarily of chromaffin cells, it secretes catecholamines—mainly epinephrine and norepinephrine—into the bloodstream in response to stress, orchestrating the fight-or-flight response.

Anatomy and location

The adrenal medulla lies at the core of the triangular or crescent-shaped adrenal gland, which is encased within the renal fascia alongside the superior pole of each kidney. It is surrounded by the adrenal cortex, from which it is separated by an irregular boundary. The entire gland receives its arterial blood supply from branches of the inferior phrenic artery, the aorta, and the renal artery, specifically the superior, middle, and inferior suprarenal arteries. Venous drainage is primarily via the suprarenal veins, with the right draining into the inferior vena cava and the left into the left renal vein or the inferior phrenic vein. The medulla itself is richly innervated by preganglionic sympathetic fibers originating in the intermediolateral nucleus of the spinal cord, which travel through the splanchnic nerves to synapse directly on chromaffin cells.

Development and histology

The adrenal medulla derives from neural crest cells, which are ectodermal in origin, unlike the mesodermal adrenal cortex. These precursor cells migrate during embryogenesis to invade the developing cortical tissue, eventually forming the central medulla. Histologically, the medulla consists of clusters and cords of large, granular chromaffin cells (or pheochromocytes) named for their affinity for chromic acid salts, which stain them a brownish color. These cells are essentially modified postganglionic neurons that lack axons and instead release their products directly into capillaries. The tissue is highly vascularized with an extensive network of sinusoidal capillaries and contains occasional ganglion cells. The presence of tyrosine hydroxylase is a key histological marker for these catecholamine-producing cells.

Physiology and function

The primary physiological role of the adrenal medulla is to amplify and disseminate the sympathetic nervous system response via the circulatory system. Upon stimulation by acetylcholine from preganglionic fibers, chromaffin cells undergo exocytosis, releasing stored catecholamines into the systemic circulation. This endocrine response is integral to the fight-or-flight response, preparing the body for acute stress. The released hormones increase cardiac output by acting on the sinoatrial node and myocardium, dilate airways in the lungs, shunt blood from the skin and digestive system to skeletal muscle, and promote glycogenolysis in the liver and skeletal muscle. This systemic effect contrasts with the localized, synaptic actions of the sympathetic nervous system proper.

Hormones and neurotransmitters

The adrenal medulla synthesizes and secretes the catecholamines epinephrine (adrenaline), norepinephrine (noradrenaline), and, to a minor extent, dopamine. The biosynthesis pathway involves the enzymes tyrosine hydroxylase, aromatic L-amino acid decarboxylase, dopamine beta-hydroxylase, and phenylethanolamine N-methyltransferase (PNMT), with PNMT being largely unique to the medulla and induced by cortisol from the adjacent cortex. Epinephrine is the major secretory product (approximately 80%), exerting its effects by binding to adrenergic receptors (alpha and beta subtypes) on target tissues. Norepinephrine acts both as a hormone from the medulla and as the primary neurotransmitter of postganglionic sympathetic neurons. These hormones are stored in chromaffin granules alongside other constituents like ATP and chromogranin A.

Clinical significance

Dysfunction of the adrenal medulla is primarily associated with tumors of chromaffin cells, known as pheochromocytomas, which can cause paroxysmal or sustained hypertension due to excessive catecholamine secretion. Diagnosis often involves measuring metabolites like vanillylmandelic acid (VMA) or metanephrines in urine. Surgical resection, often following pre-operative management with alpha blockers like phenoxybenzamine, is the definitive treatment. Neuroblastoma, a common pediatric cancer arising from neural crest cells, can also originate in the medulla. Furthermore, the adrenal medulla can be affected in systemic conditions like multiple endocrine neoplasia type 2 (MEN 2), associated with mutations in the RET proto-oncogene. Insufficiency of the medulla is rare but can occur in contexts like bilateral adrenalectomy.

Comparative anatomy

In many fish and some amphibians, the chromaffin tissue and steroidogenic cortical tissue are anatomically separate, with chromaffin cells often located near the kidneys or within the sympathetic ganglia. In birds and reptiles, the tissues intermingle but remain distinct. The intimate association of the cortex and medulla into a single gland, as seen in mammals, is considered an evolutionary adaptation for the cortisol-mediated induction of PNMT, allowing for high epinephrine production. This integrated structure is a hallmark of mammalian adrenal glands and is thought to support more complex and sustained stress responses. The hagfish possesses clusters of chromaffin cells along its cardinal veins, representing a primitive form of the system.

Category:Endocrine system Category:Anatomy of the abdomen